In road surfaces and at entrances to aboveground utilities, there are formed access holes for accessing the underground and aboveground utilities, such as water, sewerage, gas, electricity, and communication utilities. The access hole is openable and closable by means of a cover assembly. The cover assembly is composed of a support frame fixed to the access hole and a cover removably fitted into the central hole of the support frame. The support frame and the cover generally utilized are ones that have slopes. The support frame with a slope has a central hole whose top portion is formed into an inverted cone (inverted trapezoid in vertical section) with a diameter decreasing downward in the vertical direction of the support frame, whereas the cover with a slope is formed into an inverted cone that is complementary to the top portion of the central hole of the support frame. In the cover assembly shown in FIG. 6, the support frame 20 is provided in its inner periphery with a shoulder 25 that is surmounted by the cover 10. As described in Examined Japanese Utility Model Publication Nos. 57-58296 and 57-46459, the cover and the support frame may be coupled to each other with a hinge or chain.
In case that a manhole opening as an access hole is opened, the worker slightly lifts the cover 10 from the support frame 20 with the aid of a crowbar and pulls out the cover 10 onto the road surface by the side of the manhole. In case that the manhole is closed, the cover 10 on the road surface is pushed diagonally downward into the central hole of the support frame 20 to be seated in the support frame. When being pushed into the support frame, however, the cover occasionally tilts greatly in the support frame. If the cover is further pushed in the support frame while being greatly inclined as mentioned, the edge of the cover 10 is brought into contact with the inner peripheral surface 26 of the support frame 20 as illustrated in FIGS. 5 and 6, so that the cover cannot be set in the support frame. If this happens, the worker has to pull out the cover from the support frame and redo the cover-closing work. Since the cover is heavy in general, the redo of the cover-closing work requires energy and time.